Fine carbon fiber, fine short carbon fiber, and manufacturing method for said fibers

ABSTRACT

A novel fine carbon fiber is produced by vapor growth, in which a graphite-net plane consisting of carbon atoms alone forms a temple-bell-shaped structural unit comprising closed head-top part and body-part with open lower-end, where an angle θ formed by a generatrix of the body-part and a fiber axis is less than 15°, 2 to 30 of the temple-bell-shaped structural units are stacked sharing a central axis to form an aggregate, and the aggregates are connected in head-to-tail style with a distance, thereby forming a fiber. Furthermore, a fine short carbon fibers with excellent dispersibility can be obtained by shortening the fine carbon fiber.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP2009/054210, filed Mar. 5, 2009,designating the U.S., and published in Japanese as WO2009/110570 on Sep.11, 2009, which claims priority to Japanese Patent Application No.2008-055819, filed Mar. 6, 2008; and to Japanese Patent Application No.2008-171186, filed Jun. 30, 2008, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a fine carbon fiber and a fine shortcarbon fiber, which are highly conductive, as well as a process forefficiently manufacturing such a carbon fiber and a short carbon fiber.In particular, the invention relates to a process for manufacturing afine carbon fiber by vapor growth using a catalyst, as well as a processfor manufacturing a fine short carbon fiber prepared by applying shearstress to the above fine carbon fiber for further shortening.

BACKGROUND ART

Fine carbon fibers represented by, for example, a cylindrical tube type,a fish bone type (cup stack type) and a card-shaped (platelet) type areexpected to be used in various applications because of their shape andmorphology. In particular, a cylindrical tube type fine carbon fiber(carbon nanotube) has attracted the attention as a next-generationconducting material because it is excellent in properties such asstrength and electric conductivity in comparison with conventionalcarbon materials.

A multilayer carbon nanotube (multilayer concentric cylindrical)(non-fish bone) is described in, for example, Japanese publication ofexamined application No. H03-64606, Japanese publication of examinedapplication No. H03-77288, Japanese Laid-Open publication No. H09-502487and Japanese Laid-Open publication No. 2004-299986.

A fish bone type carbon fiber [cup stack type carbon fiber] is describedin, for example, U.S. Pat. No. 4,855,091, M. Endo, Y. A. Kim et al.,{Appl. Phys. Lett., vol. 80 (2002) 1267-}, Japanese Laid-Openpublication No. 2003-073928 and Japanese Laid-Open publication No.2004-360099. This structure is a stacked open-cup shape.

A platelet type carbon nanofiber (card-shaped) is described in, forexample, H. Murayama and T. Maeda {Nature, vol. 345 [No. 28] (1990) 791to 793}, and Japanese Laid-Open publication No. 2004-300631.

Conventionally known processes for manufacturing a fine carbon fiberrepresentatively exemplified by carbon nanotube include an arcdischarge, a vapor growth, a laser and a template methods. Among these,vapor growth using catalyst particles attracts attention as aninexpensive synthetic method, but is not established in a large scale.Furthermore, carbon nanotube produced is a less crystallineinhomogeneous fiber, and therefore, graphitization is necessary whenhigh conductivity is required.

For example, Japanese unexamined patent application publication No.H09-502487 (Patent Reference No. 1) has described that a carbon fibrilmaterial (cylindrical tube type) prepared as described in Japaneseunexamined patent application publication No. H02-503334 or JapaneseLaid-Open publication No. S62-500943 as a prior art has a graphite planegap (d002) of 0.354 nm as determined by XRD (X-ray diffraction) and isinsufficiently crystalline and less conductive. Furthermore, it has beendescribed that by treating this fibril material at 2450° C., a graphiteplane gap (d002) becomes 0.340 nm and a fibril material with improvedcrystallinity is obtained.

In carbon nanotube (multilayer concentric cylindrical type), agraphite-net plane is parallel to a fiber axis, along which electronsflow, so that conductivity is satisfactory in a long-axis direction in asingle fiber. However, in terms of conductivity between adjacent fibers,jumping effect by n-electron emission (tunnel effect) cannot occurbecause a side peripheral surface consists of a cylindrical closedgraphite-net plane. There is, therefore, a problem that in a compositewith a polymer using carbon nanotube as a conductive filler, if contactbetween fibers is inadequate, sufficient conductivity is not obtained.

Furthermore, since a cylindrical graphite-net plane in carbon nanotubehaving this structure consists of a SP2-bond carbon cylinder, it isdifficult to cleave strong carbon SP-2 bond by an industrial methodgenerally used (ball mill, bead mill, or the like) for furthershortening the fiber to give an industrially available fine short carbonfiber without generating structural defects in the fiber surface.

On the other hand, in a fish bone or platelet type (card-shaped) finefiber, an open end of a graphite-net plane is exposed in a sideperipheral surface, so that conductivity between adjacent fibers isimproved in comparison with carbon nanotube. However, it has a stackstructure where C-axis in the graphite-net plane is oblique orperpendicular to the fiber axis direction, so that conductivity in afiber long-axis direction in a single fiber is reduced.

In terms of fiber shortening, a fish bone type carbon fiber has astructure of stacked corn-shaped carbon fundamental planes which areoblique to the fiber axis direction as described in Japanese Laid-Openpublication No. 2004-241300 and allows for layer peeling between carbonfundamental planes or interlayer slipping, so that fibers can be easilyfurther shortened. However, since conductivity in the fiber axisdirection is extremely low as described above, not only a fish bone typecarbon fiber but also a further shortened fiber are unsuitable as aconductive material.

A platelet type (card-shaped) also has a basic structure where stackedcarbon fundamental plane disks are perpendicular to the fiber axis as inthe fish bone type carbon fiber [cup stack type carbon fiber], and thus,can be easily further shortened, but not only a platelet type carbonnanofiber but also a further shortened fiber are unsuitable as aconductive material because of the reason as in the fish bone typecarbon fiber.

In addition to the above structures, Japanese Laid-Open publication No.2006-103996 (Patent Reference No. 2) has disclosed a fiber structurecomprising a structural unit containing a nitrogen atom chemicallybonded to a carbon atom in a core of a crystal lattice and composed of atemple-bell-shaped multilayer material in which one end is opened whilethe other end is closed, wherein the closed end of one unit is insertedto the open end of another unit, as well as a manufacturing processtherefor. However, this fiber contains a nitrogen atom chemically bondedto a carbon atom in a graphite-net plane, so that structural distortionis introduced in the graphite-net plane, leading the problem of poorcrystallinity, that is, lower conductivity.

Applied Physics A 2001 (73) 259-264 (Ren Z. F. et al) (Non-patentReference No. 1) has also described a fiber structure called“bamboo-structure” similar to that in Patent Reference No. 2 (JapaneseLaid-Open publication No. 2006-103996) described above. This structureis synthesized by vapor growth at 750° C. using a silica-supported ironcatalyst and a mixed gas of 20 vol % of acetylene/80 vol % of ammonia.Although chemical composition analysis for the fiber structure is notdescribed at all in this reference, nitrogen which are not inert arecontained in a raw material in a very high concentration (59 wt %), sothat the fiber structure would also contain a nitrogen atom chemicallybonded to a carbon atom, leading to structural disturbance. Furthermore,since a ratio of a product weight to a catalyst weight is assignificantly low as about 6, the growth of fiber is insufficient,leading to a lower aspect ratio.

Furthermore, Carbon 2003 (41) 2949-2959 (Gadelle P. et al) (Non-patentReference No. 2) has also described a structure in which graphite-netplanes constituting a fiber have corn shape whose open ends are exposedto a fiber side peripheral surface with proper distance. In thisreference, 0.2 g of a mixture of a cobalt salt and a magnesium saltcoprecipitated by citric acid is activated by H₂ and then reacted with asource gas consisting of CO and H₂ (H₂ concentration: 26 vol %) to give4.185 g of a product. However, in a fiber structure obtained by thisprocess, a corn-shaped side peripheral surface forms an angle of about22° with a fiber axis, that is, it is considerably oblique to the fiberaxis. Thus, in terms of conductivity in a long-axis direction of asingle fiber, there is a problem as described above for the fish bonecarbon fiber. Furthermore, since an aspect ratio is lower due toinsufficient fiber growth, it is difficult to give conductivity orreinforcement to a composite with a polymer. Furthermore, since a ratioof a product weight to a catalyst weight is as low as 21, the process isinefficient as a manufacturing process and impurities are contained in alarge amount, leading to limited number of applications.

There have been fine carbon fibers having various structures asdescribed above and manufacturing processes therefor, but there are fewsuggestions for a process for further shortening such a fine carbonfiber. Examples are as follows.

(1) Fibers are cut by applying current within a scanning tunnelmicroscope (STM) (Non-patent Reference No. 3). In this method, fibersare cut one by one, and therefore, the method is not suitable forlarge-scale production.

(2) Using a mixed acid of an acid and nitric acid, oxidativedecomposition and ultrasonic cutting are simultaneously conducted(Non-patent Reference No. 4). This method has a problem that a carbonwall is significantly damaged. This method also has a problem of yieldreduction due to oxidation.

(3) Ball mill cutting (Non-patent Reference No. 5). In this method, theside surface of a fiber is considerably damaged and at the same time,the fiber is contaminated by impurities from a ball mill.

(4) Cutting by heating after fluorination (Non-patent Reference No. 6).This method also has a problem in an yield because a fluorinated moietyis eliminated, and an apparatus becomes larger.

(5) Cutting in a solvent using a high-speed rotation disperser(Non-patent Reference No. 7). This method is convenient, but is notsuitable for large-scale production.

List of Documents

Patent Reference No. 1: Japanese unexamined patent applicationpublication No. H09-502487.

Patent Reference No. 2: Japanese Laid-Open publication No. 2006-103996.

Non-patent Reference No. 1: Applied Physics A 2001 (73) 259-264 (Ren Z.F. et al).

Non-patent Reference No. 2: Carbon 2003 (41) 2949-2959 (Gadelle P. etal.).

Non-patent Reference No. 3: Liesbeth C. Venema et al., Appl. Phys. Lett.71, 2629 (1997).

Non-patent Reference No. 4: J. Liu et al., Science 280, 1253 (1998).

Non-patent Reference No. 5: N. Pierard et al., Chem. Phys. Lett. 335, 1(2001).

Non-patent Reference No. 6: Z. Gu et al., Nano Lett. 2, 1009 (2002).

Non-patent Reference No. 7: Text in the Seminar for Solubilization andDispersion of Carbon Nanotube in a Polymer/Solvent (Kazuyuki Taji,Technical Information Institute Co., Ltd., Mar. 31, 2006).

Problem to be Solved by the Invention

As described above, a conventional fine carbon fiber such as carbonnanotube has a problem in balance between conductivity in a long-axisdirection in a single fiber and conductivity between adjacent fibers.Furthermore, there have been established no efficient manufacturingprocesses giving a product with stable quality, and there remainsproblems for commercial application in terms of a cost and technology.On the other hand, current granular carbon black cannot be satisfactoryin the aspects of performance and functions in forming composite(s) withpolymer(s).

A conventional fine carbon fiber such as carbon nanotube has a furtherproblem of poor dispersibility. For example, while carbon nanotube issynthesized by a gas-phase method which is believed to be suitable forlarge-scale production, it forms a secondary structure in which fibersare intricately intertwined to each other, so that when it is mixedwith, for example, a resin, it exhibits poor dispersibility.

An objective of the present invention is to provide a fine carbon fiberand/or a fine short carbon fiber that have/has improved dispersibilityand kneading properties in forming a composite with a polymer and/or apowder, which in turn attains a composite having excellentprocessability, and allows a composite exhibiting excellent functionssuch as conductivity, thermal conductivity, slide properties andreinforcement, as well as a process for efficiently manufacturing them.

Means for Solving Problem

The present invention relates to the following items. Herein, the term,“fine carbon fiber” refers to connected carbon fibers which is preparedby vapor growth described later, as schematically shown in FIG. 2. Theterm, “fine short carbon fiber” refers to a carbon fiber produced bycutting some connections of a “fine carbon fiber” by applyingshear-stress, as shown in FIGS. 10 and 11.

-   1. A fine carbon fiber produced by vapor growth, wherein

a graphite-net plane consisting solely of carbon atoms forms atemple-bell-shaped structural unit comprising closed head-top part andbody-part with open lower-end, where an angle θ formed by a generatrixof the body-part and a fiber axis is less than 15°,

2 to 30 of the temple-bell-shaped structural units are stacked sharing acommon central axis to form an aggregate, and

the aggregates are connected in head-to-tail style with a distance toform the fiber.

-   2. The fine carbon fiber according to the above item 1, wherein an    end of the body-part of the aggregate has an outer diameter D of 5    to 40 nm and an inner diameter d of 3 to 30 nm, and an aspect ratio    (L/D) of the aggregate is 2 to 30.-   3. The fine carbon fiber according to the above item 1 or 2, wherein    an ash content is 4% by weight or less.-   4. The fine carbon fiber according to any one of the above items 1    to 3, wherein a peak half width W (unit: degree) of 002 plane in the    fine carbon fiber is 2 to 4 as determined by X-ray diffractometry.-   5. The fine carbon fiber according to any one of the above items 1    to 4, wherein a graphite plane gap (d002) of the fine carbon fiber    is 0.341 to 0.345 nm as determined by X-ray diffractometry.-   6. A process for manufacturing a fine carbon fiber comprising    feeding a mixed gas containing CO and H₂ onto a catalyst containing    a spinel-type oxide of cobalt, in which magnesium is contained by    substitution forming solid solution, to initiate a reaction and    growing the fine carbon fiber.-   7. The process for manufacturing a fine carbon fiber according to    the above item 6, wherein when the spinel-type oxide is represented    by Mg_(x)Co_(3-x)O_(y), “x” which is a solid solution range of    magnesium is 0.5 to 1.5.-   8. The process for manufacturing a fine carbon fiber according to    the above item 6 or 7, wherein in the spinel-type oxide, a crystal    lattice constant “a” (cubic system) is 0.811 to 0.818 nm as    determined by X-ray diffraction measurement.-   9. The process for manufacturing a fine carbon fiber according to    any one of the above items 6 to 8, wherein a volume ratio of CO/H₂    in the mixed gas is within the range of 70/30 to 99.9/0.1 and a    reaction temperature is within the range of 400 to 650° C.-   10. A fine short carbon fiber prepared by shortening a fine carbon    fiber produced by vapor growth, wherein a graphite-net plane forms a    temple-bell-shaped structural unit comprising closed head-top part    and body-part with open lower-end, 2 to 30 of the temple-bell-shaped    structural units are stacked sharing a central axis to form an    aggregate, and one to several tens of the aggregates are connected    in head-to-tail style.-   11. The fine short carbon fiber according to the above item 10,    wherein the generatrix of the body-part and the fiber axis form an    angle θ of less than 15°.-   12. The fine short carbon fiber according to the above item 10 or    11, wherein an end of the body-part of the aggregate has an outer    diameter D of 5 to 40 nm and an inner diameter d of 3 to 30 nm, and    an aspect ratio (L/D) of the aggregate is 2 to 30.-   13. The fine short carbon fiber according to any one of the above    items 10 to 12, wherein an ash content is 4% by weight or less.-   14. The fine short carbon fiber according to any one of the above    items 10 to 13, wherein a peak half width W (unit: degree) of 002    plane in the fine carbon fiber is 2 to 4 as determined by X-ray    diffractometry.-   15. The fine short carbon fiber according to any one of the above    items 10 to 14, wherein a graphite plane gap (d002) of the fine    carbon fiber is 0.341 to 0.345 nm as determined by X-ray    diffractometry.-   16. A fine short carbon fiber produced by shortening the carbon    fiber according to any one of the above items 1 to 5 by applying    shear stress.-   17. A process for manufacturing a fine short carbon fiber,    comprising preparing a fine carbon fiber by the manufacturing    process according to any one of the above items 6 to 9 and then    applying shear stress to the fiber for shortening.-   18. The use of the fine short carbon fiber according to any one of    the above items 10 to 16 as an electrical conductive material, an    electrical conductivity assisting material, a heat-conducting    material, a slide material or a polishing material.

Effect of the Invention

A fine carbon fiber of the present invention has a fiber structure inwhich an open end of a graphite-net plane constituting the fiber isexposed in the side peripheral surface of the fiber with an appropriatedistance and the side peripheral surface consisting of the graphite-netplane forms a small angle with the fiber axis. The bond of this finecarbon fiber in the direction of the fiber axis in thetemple-bell-shaped structural unit aggregate is a continuous carbon SP2bond, whose bonding strength is large and good conductivity can beachieved in the fiber axis direction. Furthermore, the fiber has regionswhere open ends of the graphite-net planes are exposed in the sideperipheral surface of the fiber with an appropriate distance at afrequency in terms of aspect ratio of about 2 to 150 along the fiberaxis direction of this carbon fiber. Thus, conductivity in a long-axisdirection in a single fiber and conductivity between adjacent fibers canbe well balanced.

Furthermore, in the region around open-end of the temple-bell-shapedstructural unit aggregate, a bonding part is formed with carbonfundamental planes that are oblique to the fiber axis. In other words,this bonding part is a part where temple-bell-shaped structural unitaggregates are bonded each other mainly by the bonding between thecarbon fundamental planes, that is, weak bonding force of van der Waalsforce. Therefore, when shear stress is applied to this part, it caneasily cause sliding between the carbon fundamental planes, so that thetemple-bell-shaped structural unit aggregate slides off or is pulled outof the bonding part, resulting in cutting of the fiber.

A method for shortening the fine carbon fiber into the fine short carbonfiber is, as a procedure, the same as the ball mill cutting methoddescribed in (3) as a conventional fiber-shortening method. However, ina conventional fine carbon fiber, the substantially whole fiber isconnected by carbon SP2 bond, and therefore, cleaving this bond requiresenormous amount of energy and the outer wall of the fiber after cuttingis considerably damaged. In contrast, a fine carbon fiber of the presentinvention has a structure where temple-bell-shaped structural unitaggregates are bonded via van der Waals force, and therefore, thebonding part can be cleaved with a relatively smaller energy andfurthermore, the fiber can be shortened by a small energy, so that thefine short carbon fiber obtained can be undamaged. Furthermore, fibershortening with a small energy means small impact on a ball mill vesseland balls and thus, it is also a distinctive feature that contaminationdue to scraping of such vessel or balls is reduced. This is confirmed bythe facts that, in the fibers described in Table 2 (Reference Example B1and Example B1-6), a carbon layer gap as determined by XRD is littlechanged before and after fiber shortening, that true specific gravity issubstantially unchanged, and that a surface area does not increasecorresponding to fiber shortening.

Furthermore, according to the process for manufacturing a fine carbonfiber, a fine carbon fiber less contaminated with impurities can beefficiently produced, so that a fine short carbon fiber prepared byapplying shear stress to the above fiber is also highly pure and fibershortening, that is, separation between bonding sites, does not requiremuch energy, resulting in less contamination by impurities from a ballmill and the like. Furthermore, the use of a fine short carbon fiber ofthe present invention reduces energy required for homogeneously mixingthe fiber with a resin or a powder. For example, since mixing with aresin does not require a large shear force at a high temperature,manufacturing of a conductive composite which keeps characteristic resinproperties can be obtained. In mixing with solid materials, a conductivecomposition keeping characteristic solid properties can be manufacturedwithout damaging the crystallinity of the solid materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a drawing schematically showing a minimal structural unit(temple-bell-shaped structural unit) constituting a fine carbon fiber;and FIG. 1( b) is a drawing schematically showing an aggregateconsisting of 2 to 30 stacked temple-bell-shaped structural units.

FIG. 2( a) is a drawing schematically showing connecting aggregates witha certain distance to form a fiber; FIG. 2( b) is a drawingschematically showing curved connection when aggregates are connectedwith a certain distance.

FIG. 3 is a TEM photographic image of a fine carbon fiber prepared inExample A1.

FIG. 4 is a TEM photographic image of a fine carbon fiber prepared inExample A2.

FIG. 5 is a TEM photographic image of a fine carbon fiber prepared inExample A3.

FIG. 6 is a TEM photographic image of a fine carbon fiber prepared inReference Example A1.

FIG. 7 is a graph summarizing the evaluation experiment results.

FIG. 8 is a TEM photographic image of a fine carbon fiber prepared inExample A3.

FIG. 9 a schematic drawing showing that a fine carbon fiber is pulledout to form a fine short carbon fiber by shear stress.

FIG. 10 is a TEM image of a fine short carbon fiber shortened in ExampleB1-6.

FIG. 11 is a TEM image of a fine short carbon fiber shortened inExamples B1-6 as shown in FIG. 10.

FIG. 12 is a SEM image showing dispersion of a fine short carbon fibershortened in Example B1-6 in carbon-coated LiFePO₄.

FIG. 13 is a SEM image showing dispersion of a fine carbon fiber ofReference Example B1 before fiber-shortening in carbon-coated LiFePO₄.

FIG. 14 is a SEM image showing dispersion of acetylene black incarbon-coated LiFePO₄ in Comparative Example B2.

FIG. 15 is a SEM image of dispersion of Ketjen black in carbon-coatedLiFePO₄ in Comparative Example B3.

FIG. 16 shows variation in particle size distribution (fiber-lengthdistribution) by ball milling (a; before ball milling, b; 6 hour ballmilling, c; 12 hour ball milling, and d; 24 hour ball milling).

EXPLANATION OF THE SYMBOLS

-   11: structural unit-   12: head-top part-   13: body-part-   21, 21 a, 21 b, 21 c: aggregate

BEST MODE FOR CARRYING OUT THE INVENTION

A fine carbon fiber and a fine short carbon fiber of the presentinvention has a temple-bell-shaped structure as shown in FIG. 1( a) as aminimal structural unit. A temple bell is commonly found in Japanesetemples, which has a relatively cylindrical-shaped body-part, which isdifferent from a Christmas bell that is very close to cone-shape. Asshown in FIG. 1( a), a structural unit 11 has a head-top part 12 and abody-part 13 having an open end like a temple bell and approximately hasa shape as a body of rotation formed by rotation about a central axis.The structural unit 11 is constituted by a graphite-net plane consistingsolely of carbon atoms, and the circumference of the open-end of thebody-part is the open end of the graphite-net plane. Here, although thecentral axis and the body-part 13 are, for convenience, indicated by astraight line in FIG. 1( a), they are not necessarily straight, but maybe curved as shown in FIGS. 3, 8, 10 and 11 described later.

The body-part 13 is gradually enlarged toward the open-end side, and asa result, the generatrix of the body-part 13 is slightly oblique to thecentral axis of the temple-bell-shaped structural unit and an angleformed θ by these is less than 15°, more preferably 1°<θ<15°, furtherpreferably 2°<θ<10°. With an excessively large θ, a fine fiberconstituting from the structural units has a structure like a fish bonecarbon fiber, leading to deterioration in conductivity in a fiber axisdirection. On the other hand, with a smaller θ, it has a structure likea cylindrical tube and thus the open end of the graphite-net planeconstituting the body-part in the structural unit are less exposed inthe outer circumference surface of the fiber, leading to deteriorationin conductivity between adjacent fibers.

A fine carbon fiber and a fine short carbon fiber of the presentinvention have defects and irregular disturbances, but when their shapeis observed as a whole neglecting such irregularity, it can be said thatthey have a temple-bell-shaped structure where the body-part 13 isgradually enlarged toward the open end side. In terms of a fine shortcarbon fiber and a fine carbon fiber of the present invention, the abovedescription does not mean that θ is within the above range in all parts,but means that when the structural unit 11 is observed as a wholeneglecting defects and irregular parts, θ generally is within the aboverange. Therefore, in determination of θ, it is preferable to eliminatean area near the head-top part 12 where a thickness of the body-partirregularly varies. More specifically, for example, when a length of atemple-bell-shaped structural unit aggregate 21 (see, the descriptionbelow) is “L” as shown in FIG. 1( b), θ may be measured at three points(¼)L, (½)L and (¾)L from the head-top part side and an average of themeasured values is determined and the average may be regarded as θ forthe whole structural unit 11. “L” is ideally measured in a straightline, but actually, the body-part 13 is often curved, and therefore, itcan be measured along the curve in the body-part 13 to give asubstantially more real value.

When produced as a fine carbon fiber (the same goes for a fine shortcarbon fiber), the head-top part has a shape which is smoothly connectedto the body-part and convexly curved to the upper side (in the figure).A length of the head-top part is typically about “D” (see FIG. 1( b)) orless, sometimes about “d” (see FIG. 1( b)) or less, wherein “D” and “d”will be described for a temple-bell-shaped structural unit aggregate.

Furthermore, as described later, active nitrogen is not used as astarting material, so that other atoms such as nitrogen are notcontained in the graphite-net plane of the temple-bell-shaped structuralunit. Thus, the fiber exhibits excellent crystallinity.

In a fine carbon fiber and a fine short carbon fiber of the presentinvention, as shown in FIGS. 1( b), 2 to 30 of such temple-bell-shapedstructural units are stacked sharing a central axis, to form atemple-bell-shaped structural unit aggregate 21 (hereinafter, sometimessimply referred to as an “aggregate”). The stack number is preferably 2to 25, more preferably 2 to 15.

An outer diameter “D” of the body-part of the aggregate 21 is 5 to 40nm, preferably 5 to 30 nm, further preferably 5 to 20 nm. A diameter ofa fine fiber increases as “D” increases, so that in a composite with apolymer, a large amount needs to be added for giving particularfunctions such as conductivity. On the other hand, as “D” decreases, adiameter of a fine fiber decreases, so that fibers tend to more stronglyagglomerate each other, leading to, for example, difficulty indispersing them in preparation of a composite with a polymer. Abody-part outer diameter “D” is determined preferably by measuring it atthree points (¼)L, (½)L and (¾)L from the head-top part of the aggregateand calculating an average. Although FIG. 1( b) shows a body-part outerdiameter “D” for convenience sake, an actual “D” is preferably anaverage of the measured values at the above three points.

An inner diameter “d” of the body-part of the aggregate is 3 to 30 nm,preferably 3 to 20 nm, further preferably 3 to 10 nm. Again, a body-partinner diameter “d” is determined preferably by measuring it at threepoints (¼)L, (½)L and (¾)L from the head-top part of thetemple-bell-shaped structural unit aggregate and calculating an average.Although FIG. 1( b) shows a body-part inner diameter “d” for conveniencesake, an actual “d” is preferably an average of the measured values atthe above three points.

An aspect ratio (L/D) calculated from a length “L” of the aggregate 21and a body-part outer diameter “D” is 2 to 150, preferably 2 to 30, morepreferably 2 to 20, further preferably 2 to 10. With a larger aspectratio, a fiber formed has a structure of a more cylindrical tube andconductivity in a fiber axis direction in a single fiber is improved,but the open ends of the graphite-net planes constituting the body-partof the structural units are less frequently exposed in thecircumferential surface of the fiber, leading to deterioration inconductivity between adjacent fibers. On the other hand, with a smalleraspect ratio, the open ends of the graphite-net planes constituting thebody-part of the structural units are more frequently exposed in thecircumferential surface of the fiber, so that conductivity betweenadjacent fibers can be improved, but a fiber circumferential surface isconstituted by a number of connected short graphite-net planes in afiber axis direction, leading to deterioration in conductivity in afiber axis direction in a single fiber.

A fine carbon fiber and a fine short carbon fiber of the presentinvention share an essentially common configuration for atemple-bell-shaped structural unit and a temple-bell-shaped structuralunit aggregate, but a fiber length is different as described below.

First, a fine carbon fiber of the present invention is formed byconnecting the aggregates in a head-to-tail style as shown in FIG. 2(a). A head-to-tail style means that in a configuration of the finecarbon fiber, a bonding site between adjacent aggregates is formed froma combination of the head-top part (head) of one aggregate and the lowerend (tail) of the other aggregate. As a specific style of bonding thepart, the head-top part of the outermost temple-bell-shaped structuralunit in the second aggregate 21 b is inserted into the inner part of theinnermost temple-bell-shaped structural unit at a lower opening of afirst aggregate 21 a; and furthermore, the head-top part of a thirdaggregate 21 c is inserted into the lower opening of a second aggregate21 b, and a number of such combinations are serially connected to form afiber.

Each bonding part forming one fine fiber of the fine carbon fibers doesnot have structural regularity; for example, a length of a bonding partbetween a first aggregate and a second aggregate in a fiber axisdirection is not necessarily equal to a length of a bonding part betweenthe second aggregate and a third aggregate. Furthermore, as shown inFIG. 2( a), two aggregates bonded share a common central axis and may beconnected in a straight line, but as in the temple-bell-shapedstructural unit aggregates 21 b and 21 c shown in FIG. 2( b), they maybe bonded without sharing a central axis, resulting in a curvedstructure in the bonding part. A length “L” of the temple-bell-shapedstructural unit aggregate is approximately constant in each fiber.However, since in vapor growth, starting materials and byproduct gaseouscomponents and a catalyst and a solid product component exist inmixture, a temperature distribution may occur in a reaction vessel; forexample, a local site at a temporarily higher temperature generatesdepending on a flowing state of the above heterogeneous reaction mixtureof a gas and a solid during an exothermic carbon precipitating reaction,possibly resulting in variation in a length “L” to some extent.

In the fine carbon fiber thus constituted, at least some of the openends of the graphite-net planes in the lower end of thetemple-bell-shaped structural units are exposed in the fibercircumferential surface, depending on a connection distance of theaggregates. Consequently, without conductivity in a fiber axis directionin a single fiber being deteriorated, conductivity between adjacentfibers can be improved due to jumping effect by n-electron emission(tunnel effect) as described above. Such a fine carbon fiber structurecan be observed by a TEM image. Furthermore, it can be believed that theeffects of a fine carbon fiber of the present invention are littleaffected by curving of the aggregate itself or curving of the connectionpart of the aggregates. Therefore, parameters associated with astructure can be determined by observing an aggregate having arelatively straight part in a TEM image, as the structural parameters(θ, D, d, L) for the fiber.

Next, a fine short carbon fiber of the present invention is prepared byfurther shortening the fine carbon fiber thus formed. Specifically,shear stress is applied to the fine carbon fiber, to cause slidingbetween graphite fundamental planes in the aggregate bonding part, sothat the fine carbon fiber is shortened at some of the bonding parts ofthe aggregates to give a shorter fiber. The fine short carbon fiberformed by such fiber shortening is as short as a fiber length of 1 toabout several ten aggregates (that is, 100 or less, up to about 80,preferably up to about 70), preferably one to 20 aggregates which areconnected. An aspect ratio of the aggregates in this fine short carbonfiber is about 2 to 150. An aspect ratio of the aggregates in the fineshort carbon fiber which is suitable for mixing is 2 to 50. Even whenshear stress is applied, cleavage does not occur in a fiber straightbody-part of the fiber consisting of carbon SP2 bonds in the aggregate,so that the fiber cannot be cut into a unit smaller than an aggregate.

Also in the fine short carbon fiber, since the end surface of thegraphite net is exposed, conductivity between adjacent fibers is as highas a fine carbon fiber before fiber shortening due to jumping effect byπ-electron emission (tunnel effect) as described above whileconductivity in a fiber axis in a single fiber is not deteriorated. Astructure of a fine short carbon fiber after fiber shortening asdescribed above can be observed by a TEM image (see FIGS. 10 and 11).Furthermore, it can be believed that the effects of the fine shortcarbon fiber are little affected by curving of the aggregate itself orcurving of the bonding part of the aggregates. In the fine short carbonfiber in FIG. 10, four temple-bell-shaped structural unit aggregates of4-a to 4-d are connected as shown in the figure, and for each, θ and anaspect ratio (L/D) are 4-a: θ=4.8°, (L/D)=2.5; 4-b: θ=0.5°, (L/D)=2.0;4-c: θ=4.5°, (L/D)=5.0; 4-d: θ=1.1°, (L/D)=5.5. In the fine short carbonfiber in FIG. 11, four temple-bell-shaped structural unit aggregates of5-a to 5-d are connected as shown in the figure and for each, θ and anaspect ratio (L/D) are 5-a: θ=10°, (L/D)=4.3; 5-b: θ=7.1°, (L/D)=3.4;5-c: θ=9.5°, (L/D)=2.6; 5-d: θ=7.1°, (L/D)=4.3.

In XRD by Gakushin-method of a fine carbon fiber and a short carbonfiber, a peak half width W (unit: degree) of 002 plane measured iswithin the range of 2 to 4. If W is more than 4, graphite exhibits poorcrystallinity and poor conductivity. On the other hand, if W is lessthan 2, graphite exhibits good crystallinity, but at the same time,fiber diameter becomes large, so that a larger amount is required forgiving functions such as conductivity to a polymer.

A graphite plane gap d002 as determined by XRD using Gakushin-method ofa fine carbon fiber and a short carbon fiber is 0.350 nm or less,preferably 0.341 to 0.348 nm. If d002 is more than 0.350 nm, graphitecrystallinity is deteriorated and conductivity is reduced. On the otherhand, a fiber of 0.341 nm is produced in a low yield in the production.

The ash content contained in the fine carbon fiber and the short carbonfiber of the present invention is 4% by weight or less, and therefore,purification is not necessary for a common application. Generally, it is0.3% by weight or more and 4% by weight or less, more preferably 0.3% byweight or more and 3% by weight or less. The ash content is determinedfrom a weight of an oxide as a residue after combustion of 0.1 g or moreof a fiber.

A short carbon fiber of the present invention has a fiber length ofpreferably 100 to 1000 μm, more preferably 100 to 300 μm. A fine shortcarbon fiber having such a length in which a peak half width W (unit:degree) of the above 002 plane is 2 to 4 and a graphite plane gap d002is 0.350 nm or less, preferably 0.341 to 0.348 nm is a novel fiber whichis not known in the prior art.

There will be described a process for manufacturing a fine carbon fiberand a short carbon fiber of the present invention. A fine short carbonfiber of the present invention is produced by shortening a fine carbonfiber.

Process for Manufacturing a Fine Carbon Fiber

First, a process for manufacturing a fine carbon fiber is as follows.Using a catalyst which is an oxide of cobalt having a spinel typecrystal structure containing magnesium by substitution forming solidsolution, vapor growth is conducted supplying a mixed gas containing COand H₂ to the catalyst particles to produce a fine carbon fiber.

A spinel type crystal structure of cobalt where Mg is substitutedforming solid solution is represented by Mg_(x)Co_(3-x)O_(y). In thisformula, x is a number indicating substitution of Co by Mg, andnominally, 0<x<3. Furthermore, y is a number selected such that electriccharge of the whole formula becomes neutral, and is formally a number of4 or less. That is, a spinel-type oxide of cobalt Co₃O₄ containsdivalent and trivalent Co ions, and when divalent and trivalent cobaltions are represented by Co^(II) and Co^(III), respectively, a cobaltoxide having a spinel type crystal structure is represented byCo^(II)Co^(III) ₂O₄. Both sites of Co^(II) and Co^(III) are substitutedby Mg to form a solid solution. After the solid solution formation bysubstitution with Mg for Co^(III), electric charge is kept to be neutraland thus y is less than 4. However, both x and y have a value within arange that a spinel type crystal structure can be maintained.

For the use as a catalyst, a solid solution range of Mg represented by xis preferably 0.5 to 1.5, more preferably 0.7 to 1.5. A solid solutionamount as x of less than 0.5 results in poor catalyst activity, leadingto production of a fine carbon fiber in a lower yield. If x is more than1.5, it is difficult to produce a spinel type crystal structure.

A spinel-type oxide crystal structure of the catalyst can be confirmedby XRD, and a crystal lattice constant “a” (cubic system) is within therange of 0.811 to 0.818 nm, more preferably 0.812 to 0.818 nm. If “a” issmall, substitutional solid solution formation with Mg is inadequate andcatalyst activity is low. The above spinel-type oxide crystal having alattice constant larger than 0.818 nm is difficult to produce.

We suppose that such a catalyst is suitable because solid solutionformation by substitution with magnesium in the spinel structure oxideof cobalt provides a crystal structure as if cobalt is dispersedlyplaced in magnesium matrix, so that under the reaction conditions,aggregation of cobalt is inhibited.

A particle size of the catalyst can be selected as appropriate and forexample, is 0.1 to 100 μm, preferably 0.1 to 10 μm as a median diameter.

Catalyst particles are generally placed on an appropriate support suchas a substrate or a catalyst bed by an appropriate application methodsuch as spraying, for use. Spraying catalyst particles on a substrate orcatalyst bed can be conducted by directly spraying the catalystparticles or spraying a suspension of the particles in a solvent such asethanol and then drying it to spray a desired amount.

It is also preferable that catalyst particles are activated before beingreacted with a source gas. Activation is generally conducted by heatingunder a gas atmosphere containing H₂ or CO. Such activation can beconducted by diluting the above gas with an inert gas such as He and N₂as necessary. A temperature at which activation is conducted ispreferably 400 to 600° C., more preferably 450 to 550° C.

There are no particular restrictions to a reactor for vapor growth,which can be conducted using a reactor such as a fixed-bed reactor and afluidized-bed reactor.

A mixed gas containing CO and H₂ is used as a source gas to be a carbonsource in vapor-phase growth.

An addition concentration of H₂ gas {(H₂/(H₂+CO)} is preferably 0.1 to30 vol %, more preferably 2 to 20 vol %. When the addition concentrationis too low, cylindrical graphite net planes form a carbon-nanotube-likestructure parallel to a fiber axis. On the other hand, if it is morethan 30 vol %, the angle of the temple-bell-shaped structure oblique tothe fiber axis of a carbon side peripheral surface becomes larger andsimilar to a fish-bone shape, leading to lower conductivity in a fiberdirection.

The source gas can contain an inert gas. Examples of such an inert gasinclude CO₂, N₂, He and Ar. The inert gas is preferably contained insuch an amount that it does not significantly reduce a reaction rate;for example, 80 vol % or less, preferably 50 vol % or less. Furthermore,a synthetic gas containing H₂ and CO or a waste gas such as a steelconverter exhaust gas can be, as necessary, used after appropriatetreatment.

A reaction temperature for conducting vapor-phase growth is preferably400 to 650° C., more preferably 500 to 600° C. If a reaction temperatureis too low, a fiber does not grow. On the other hand, if a reactiontemperature is too high, an yield is reduced. A reaction time is, butnot limited to, for example, 2 hours or more and about 12 hours or less.

In terms of a reaction pressure, vapor-phase growth can be conducted atan ambient pressure from the viewpoint of convenience of a reactor oroperation, but as long as carbon growth of Boudouard equilibriumproceeds, the reaction can be conducted under the pressurized orreduced-pressure condition.

It has been demonstrated that according to this manufacturing processfor a fine carbon fiber, an yield of a fine carbon fiber per a unitweight of the catalyst is considerably higher than that in aconventional manufacturing process, for example, the method described inNon-patent Reference No. 2. An yield of a fine carbon fiber according tothis manufacturing process for a fine carbon fiber is 40 folds or more,for example 40 to 200 folds per a unit weight of the catalyst. As aresult, a fine carbon fiber containing reduced amount of impurities andash content as described above can be produced.

Although a process of forming the bonding part unique to the fine carbonfiber prepared by this manufacturing process for a fine carbon fiber isnot clearly understood, it is speculated that balance between exothermicBoudouard equilibrium and heat removal by source-gas flowing causesvariation of a temperature near the fine cobalt particles formed fromthe catalyst, so that carbon growth intermittently proceeds, resultingin formation of the bonding part. In other words, it is speculated thatfour processes: [1] formation of a head-top part of a temple-bell-shapedstructure, [2] growth of a body-part in the temple-bell-shapedstructure, [3] pause of growth due to temperature increase caused by theprocesses [1] and [2], and [4] cooling by a flowing gas, are repeated onfine catalyst particles, to form the bonding part unique to a finecarbon fiber structure.

Manufacturing Process for a Fine Short Carbon Fiber

As described above, a fine carbon fiber of the present invention can beproduced. Next, a fine short carbon fiber of the present invention canbe produced by separating a fine carbon fiber to shorten it. Preferably,it is prepared by applying shear stress to the fine carbon fiber.Suitable examples of a specific fiber shortening method include thoseusing a grinder, a tumbling ball mill, a centrifugal ball mill, acentrifugal planetary ball mill, a bead mill, a microbead mill, anattriter type high-speed ball mill, a rotating rod mill, a vibrating rodmill, a roll mill and a three-roll mill. The fiber shortening of thefine carbon fiber may be conducted in wet-process or dry-process. Wetfiber shortening may be conducted in the presence of either a resin or aresin and a filler. Since fine carbon fibers before fiber shorteningaggregate like a fluff ball, the presence of a small medium capable ofloosening such a ball can accelerate shredding and fiber shortening.Furthermore, coexistence of a fine filler allows for shortening of thefine carbon fiber and mixing and dispersing the filler can be conductedat the same time. An atmosphere in dry fiber shortening can be selectedfrom an inert atmosphere or an oxidative atmosphere, depending on apurpose.

The reason why the fine carbon fiber can be easily shortened by applyingshear stress is due to the structure of the fine carbon fiber.Specifically, it is because a fine carbon fiber is formed fromtemple-bell-shaped structural unit aggregates connected in ahead-to-tail style with a distance. When shear stress is applied to thefiber, the fiber is pulled to the fiber axis direction indicated by anarrow in FIG. 9, and then sliding occurs between carbon fundamentalplanes constituting a bonding part (in FIG. 9, see region A: “twosticks” shape which is Japanese katakana “ha”), and one to several tentemple-bell-shaped structural unit aggregates are pulled off at thehead-to-tail bonding parts, resulting in fiber shortening. That is, thehead-to-tail bonding part is not formed by consecutive carbon doublebonds in a fiber axis direction like a concentric fine carbon fiber, butformed by bonds mainly via van der Waals force with a lower bond energy.In Table 2 of Examples, when crystallinity is compared between a finecarbon fiber and a fine short carbon fiber of the present inventionprepared by shortening the above fine carbon fiber (Reference Example B1and Examples B1-1 to B1-6) on the basis of a carbon layer gap and a truespecific gravity, difference is not observed in carbon crystallinitybetween these. However, in comparison with the fine carbon fiber, thefine short carbon fiber of the present invention after fiber shorteninghas a larger surface area by about 2 to 5%. Increase of a surface areato this extent would be due to fiber shortening, indicating thatshortening of a fine carbon fiber is caused by the pulling-off of thetemple-bell-shaped structural unit aggregates simply from their bondingsites, while carbon crystallinity of the temple-bell-shaped structuralunit aggregates in the fine carbon fiber is not deteriorated.

A fine short carbon fiber of the present invention is effective forproviding conductivity or helping providing conductivity to variousresins or inorganic materials. In particular, regardless of a shape suchas spherical, whisker, flat and nanoparticles, it is effective forproviding conductivity or helping providing conductivity to anonconductive or less conductive inorganic material such as half metal,oxide, fluoride, nitride, carbide, boride, sulfide and hydride,particularly a solid material used as a battery material. Furthermore, afine short carbon fiber according to the present invention is useful,besides the use in providing conductivity or helping providingconductivity, as a heat-conducting material, a slide material and afiller for a reinforcement or a polishing material by forming a complexwith a resin or inorganic material, utilizing higher thermalconductivity and slide properties and higher tensile strength andelastic modulus which are peculiar to carbon having a graphitestructure.

EXAMPLES

There will be described Examples of the present invention together withComparative Examples.

Example A1

In 500 mL of ion-exchanged water were dissolved 115 g of cobalt nitrate[Co(NO₃)₂.6H₂O: molecular weight 291.03] (0.40 mol) and 102 g ofmagnesium nitrate [Mg(NO₃)₂.6H₂O: molecular weight 256.41] (0.40 mol),to prepare raw-material solution (1). Furthermore, 220 g of powderyammonium bicarbonate [(NH₄)HCO₃: molecular weight 79.06] (2.78 mol) wasdissolved in 1100 mL of ion-exchanged water, to prepare raw-materialsolution (2). Next, raw-material solutions (1) and (2) were mixed at areaction temperature of 40° C., after which the mixture was stirred for4 hours. The precipitate formed was filtered, washed and then dried.

The product was calcined and pulverized with a mortar to provide 43 g ofa catalyst. A crystal lattice constant “a” (cubic system) of the spinelstructure in this catalyst was 0.8162 nm, and a metallic element ratioin the spinel structure of the substitutional solid solution wasMg:Co=1.4:1.6.

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) wasperpendicularly fixed and in its center was placed a support of silicawool on which was then dispersed 0.9 g of the catalyst. Under Heatmosphere, the tube was heated to a furnace temperature of 550° C., andthen a mixed gas consisting of CO and H₂ (volume ratio: CO/H₂=95.1/4.9)as a source gas was fed from the bottom of the reaction tube at a flowrate of 1.28 L/min for 7 hours, to synthesize a fine carbon fiber.

An yield was 53.1 g, and an ash content was determined as 1.5% byweight. A peak half width “W” (degree) observed in XRD analysis of theproduct was 3.156 and d002 was 0.3437 nm. Furthermore, from a TEM image,parameters related to the dimensions of temple-bell-shaped structuralunits constituting the fine carbon fiber obtained and the aggregate ofthese were D=12 nm, d=7 nm, L=114 nm, L/D=9.5, θ=0 to 7° (average: about3°). A stack number of the temple-bell-shaped structural unitsconstituting the aggregate was 4 to 5. Here, D, d and θ were determinedfor three points (¼)L, (½)L and (¾)L from the head-top of the aggregate.

FIG. 3 shows a TEM image of the fine carbon fiber prepared in ExampleA1.

Example A2

In 900 mL of ion-exchanged water was dissolved 123 g of cobalt nitrate[Co(NO₃)₂.6H₂O: molecular weight 291.03] (0.42 mol) and then was added17 g of magnesium oxide [MgO: molecular weight 40.30] (0.42 mol), andthe mixture was mixed to prepare raw-material slurry (1). Furthermore,123 g of powdery ammonium bicarbonate [(NH₄)HCO₃: molecular weight79.06] (1.56 mol) was dissolved in 800 mL of ion-exchanged water, toprepare raw-material solution (2). Next, raw-material slurry (1) andraw-material solution (2) were mixed at room temperature, after whichthe mixture was stirred for 2 hours. The precipitate formed wasfiltered, washed and then dried. The product was calcined and pulverizedwith a mortar to provide 48 g of a catalyst. A crystal lattice constant“a” (cubic system) of a spinel structure in this catalyst was 0.8150 nm,and a metallic element ratio in the spinel structure of thesubstitutional solid solution was Mg:Co=1.2:1.8.

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) wasperpendicularly fixed and in its center was placed a support of silicawool on which was then dispersed 0.3 g of the catalyst. Under Heatmosphere, the tube was heated to a furnace temperature of 500° C., andthen from the bottom of the reaction tube, H₂ was fed at a flow rate of0.60 L/min for 1 hour to activate the catalyst. Then, under Heatmosphere, the tube was heated to a furnace temperature of 575° C. andthen a mixed gas consisting of CO and H₂ (volume ratio: CO/H₂=92.8/7.2)as a source gas was fed at a flow rate of 0.78 L/min for 7 hours, tosynthesize a fine carbon fiber.

An yield was 30.8 g and an ash content was 0.6% by weight. A peak halfwidth “W” (degree) observed in XRD analysis of the product was 3.141 andd002 was 0.3433 nm. Furthermore, from a TEM image, parameters related tothe dimensions of temple-bell-shaped structural units constituting thefine carbon fiber obtained and the aggregate of these were D=10 nm, d=5nm, L=24 nm, L/D=2.4, θ=1 to 14° (average: about 6°). A stack number ofthe temple-bell-shaped structural units constituting the aggregate was 4to 5. Here, D, d and θ were determined for three points (¼)L, (½)L and(¾)L from the head-top of the aggregate.

FIG. 4 shows a TEM image of the fine carbon fiber prepared in ExampleA2.

Example A3

A catalyst was prepared as described in Example A1, using 86 g ofmagnesium acetate [Mg(OCOCH₃)₂.4H₂O: molecular weight 214.45] (0.40 mol)in place of magnesium nitrate. A crystal lattice constant “a” (cubicsystem) of a spinel structure in the catalyst thus prepared was 0.8137nm, and a metallic element ratio in the spinel structure of thesubstitutional solid solution was Mg:Co=0.8:2.2.

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) wasperpendicularly fixed and in its center was placed a support of silicawool on which was then dispersed 0.6 g of the catalyst. Under Heatmosphere, the tube was heated to a furnace temperature of 500° C., andthen from the bottom of the reaction tube, H₂ was fed at a flow rate of0.60 L/min for 1 hour to activate the catalyst. Then, under Heatmosphere, the tube was heated to a furnace temperature of 590° C. andthen a mixed gas consisting of CO and H₂ (volume ratio: CO/H₂=84.8/15.2)as a source gas was fed at a flow rate of 0.78 L/min for 6 hours, tosynthesize a fine carbon fiber.

An yield was 28.2 g and an ash was 2.3% by weight. A peak half width “W”(degree) observed in XRD analysis of the product was 2.781 and d002 was0.3425 nm. Furthermore, from a TEM image, parameters related to thedimensions of temple-bell-shaped structural units constituting the finecarbon fiber obtained and the aggregate of these were D=12 nm, d=5 nm,L=44 nm, L/D=3.7, θ=0 to 3° (average: about 2°). A stack number of thetemple-bell-shaped structural units constituting the aggregate was 13.Here, D, d and θ were determined for three points (¼)L, (½)L and (¾)Lfrom the head-top of the aggregate.

FIG. 5 shows a TEM image of the fine carbon fiber prepared in ExampleA3.

Comparative Example A1

A catalyst was prepared as described in Example A1, except thatmagnesium nitrate was not used, and the amounts of powdery ammoniumbicarbonate and ion-exchanged water for dissolving the powder werechanged to 110 g and 550 mL, respectively. A crystal lattice constant“a” (cubic system) of the spinel structure in the catalyst thus obtainedwas 0.8091 nm. Using this catalyst, the procedure described in ExampleA2 was conducted, but the reaction little proceeded and a material wasrecovered at a weight substantially equal to that of the chargedcatalyst.

Reference Example A1

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) wasperpendicularly fixed and in its center was placed a support of silicawool on which was then dispersed 0.6 g of the catalyst prepared inExample A2. Under He atmosphere, the tube was heated to a furnacetemperature of 500° C., and then from the bottom of the reaction tube,H₂ was fed at a flow rate of 0.60 L/min for 1 hour to activate thecatalyst. Then, under He atmosphere, the tube was heated to a furnacetemperature of 650° C. and then a mixed gas consisting of CO and H₂(volume ratio: CO/H₂=60/40) as a source gas was fed at a flow rate of0.78 L/min for 6 hours, to synthesize a fine carbon fiber.

An yield was 11.2 g and an ash content was 6.1% by weight. A peak halfwidth “W” (degree) observed in XRD analysis of the product was 2.437 andd002 was 0.3424 nm. Furthermore, from a TEM image, parameters related tothe dimensions of temple-bell-shaped structural units constituting thefine carbon fiber obtained and the aggregate of these were D=9 nm, d=6nm, L=13 nm, L/D=1.4, θ=9 to 36° (average: about 19°). A stack number ofthe temple-bell-shaped structural units constituting the aggregate was5. Here, D, d and θ were determined for three points (¼)L, (½)L and (¾)Lfrom the head-top of the aggregate.

FIG. 6 shows a TEM image of the fine carbon fiber prepared in ReferenceExample A1.

Evaluation Experiment

In a resin vessel with a diameter of 2 cm was charged 0.5 g of a finecarbon fiber shown in Table 1, and under pressure by a press, a powderresistance was measured. Catalysts used are as follows.

Evaluation Example 1: the fine carbon fiber prepared in Example A1;

Evaluation Example 2: the fine carbon fiber prepared in Example A2;

Evaluation Example 3: a commercially available multilayer carbonnanotube (Reagent No. 677248 from Aldrich);

Evaluation Example 4: the fine carbon fiber prepared in ReferenceExample A1.

TABLE 1 Evaluation Evaluation Evaluation Example 1 Example 2 Example 3Evaluation Example 4 Press Volume Press Volume Press Volume Volumepressure resistivity pressure resistivity pressure resistivity Presspressure resistivity (MPaG) (Ω · cm) (MPaG) (Ω · cm) (MPaG) (Ω · cm)(MPaG) (Ω · cm) 1.0 0.086 1.0 0.087 0.7 0.088 0.7 0.147 1.4 0.077 1.40.079 1.1 0.082 1.1 0.130 2.0 0.069 1.8 0.071 1.5 0.076 1.6 0.116 2.70.062 2.4 0.063 2.2 0.070 2.5 0.101 3.9 0.055 3.4 0.057 3.1 0.064 3.90.087 5.8 0.048 4.9 0.050 4.7 0.058 6.1 0.074 8.5 0.042 7.1 0.044 7.20.051 10 0.063 12.5 0.038 11.2 0.038 11.3 0.045 11 0.061

As seen in FIG. 7, a low volume resistivity was obtained with an equalpress pressure in Evaluation Examples 1 and 2 in contrast to EvaluationExamples 3 and 4. This indicates that in comparison with a carbon fiberused in a cylindrical tube structure (Evaluation Example 3) or astructure like a fish bone carbon fiber (Evaluation Example 4), a finecarbon fiber of the present invention has improved conductivityperformance because it is well balanced in conductivity in a long-axisdirection in a single fiber and conductivity between adjacent fibersfrom its structural features. Thus, for example, it can exercise goodconductivity performance in a composite with a polymer.

Example A4

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) wasperpendicularly fixed and in its center was placed a support of silicawool on which was then dispersed 0.2 g of the catalyst prepared inExample A2. A source gas was a mixed gas consisting of CO, CO₂, N₂ andH₂ (volume ratio: CO/CO₂/N₂/H₂=67.4/16.1/15.3/1.2) supposing an exhaustgas from an LD (Linz-Donawitz) steel converter. The source gascorresponded the gas in which oxygen and water had been removed from theLD-steel converter exhaust gas, and a blend ratio was determined inaccordance with Carbon 1992(30), 975-979 (Ishioka M. et al.). After thetube was heated at a furnace temperature of 560° C. under He atmosphere,the above mixed gas was fed from the bottom of the reaction tube at aflow rate of 1.3 L/min, to synthesize a fine carbon fiber.

An yield was 8.4 g and an ash content was 1.7% by weight. A peak halfwidth “W” (degree) observed in XRD analysis of the product was 3.533 andd002 was 0.3440 nm. Furthermore, from a TEM image, parameters related tothe dimensions of temple-bell-shaped structural units constituting thefine carbon fiber obtained and the aggregate of these were D=12 nm, d=5nm, L=84 nm, L/D=7.0, θ=0 to 2° (average: about 1°). A stack number ofthe temple-bell-shaped structural units constituting the aggregate was13. Here, D, d and θ were determined for three points (¼)L, (½)L and(¾)L from the head-top of the aggregate.

FIG. 8 shows a TEM image of the fine carbon fiber prepared in ExampleA4.

Next, there will be described a production example of a fine shortcarbon fiber.

Example B1

In 500 mL of ion-exchanged water were dissolved 115 g of cobalt nitrate[Co(NO₃)₂.6H₂O: molecular weight 291.03] (0.40 mol) and 102 g ofmagnesium nitrate [Mg(NO₃)₂.6H₂O: molecular weight 256.41] (0.40 mol),to prepare raw-material solution (1). Furthermore, 220 g of powderyammonium bicarbonate [(NH₄)HCO₃: molecular weight 79.06] (2.78 mol) wasdissolved in 1100 mL of ion-exchanged water, to prepare raw-materialsolution (2). Next, raw-material solutions (1) and (2) were mixed at areaction temperature of 40° C., after which the mixture was mixed withstirring for 4 hours. The precipitate formed was filtered, washed andthen dried.

The product was calcined and pulverized with a mortar to provide 43 g ofa catalyst. A crystal lattice constant “a” (cubic system) of a spinelstructure in this catalyst was 0.8162 nm, and a metallic element ratioin the spinel structure of the substitutional solid solution wasMg:Co=1.4:1.6.

A quartz reaction tube (inner diameter: 75 mmφ, height: 650 mm) wasperpendicularly fixed and in its center was placed a support of silicawool on which was then dispersed 0.9 g of the catalyst. Under Heatmosphere, the tube was heated to a furnace temperature of 545° C., andthen a mixed gas consisting of CO and H₂ (volume ratio: CO/H₂=95.5/4.5)as a source gas was fed from the bottom of the reaction tube at a flowrate of 1.45 L/min for 7 hours, to synthesize a fine carbon fiber.

An yield was 56.7 g, and an ash content was determined as 1.4% byweight. A peak half width “W” (degree) observed in XRD analysis of theproduct was 3.39 and d002 was 0.3424 nm.

A fine carbon fibers thus obtained was treated by a ceramic ball millwith a diameter of 2 mm for a predetermined time to prepare a fine shortcarbon fiber. FIGS. 10 and 11 show TEM images of a fine short carbonfiber after 20 hours. From the TEM images in FIGS. 10 and 11, parametersrelated to the dimensions of temple-bell-shaped structural unitsconstituting the fine short carbon fiber thus obtained and the aggregateof these were D=10.6 to 13.2 nm, L/D=2.0 to 5.5, θ=0.5° to 10°. Here, θis an inclination average of the right and the left carbon layers to thecenter of the fiber axis in the TEM image. A stack number of thetemple-bell-shaped structural units forming the aggregate was 10 to 20.

In a vessel were mixed 100 parts by weight of carbon-coatedlow-conductivity solid LiFePO₄, 5 parts by weight of a binder PVDF, 5parts by weight of a fine short carbon fiber as an electricalconductivity assisting material and 110 parts by weight of a solventNMP, and the mixture was kneaded at 1200 rpm for 20 min using NBK-1kneader from Nippon Seiki Co., Ltd., and the kneaded slurry was appliedon a polyimide film and then dried to form a film. Table 3 shows aprocessing time of fiber shortening by a ball mill as well as athickness and a surface resistance of a film prepared using fine shortcarbon fibers (Examples B1-1 to B1-6) obtained with the processing timebeing varied. Also, Table 3 shows, for comparison, a thickness and asurface resistance of a film prepared by molding as described above,using a fine carbon fiber which was not shortened (Reference ExampleB1), acetylene black (Comparative Example B2), Ketjen black EC-600JD(Comparative Example B3) as an electrical conductivity assistingmaterial.

Table 2 shows the basic physical properties of electrical conductivityassisting materials used in Examples, Reference Examples and ComparativeExamples. In Table 2, a thickness of the stacked electrical conductingmaterial carbon layers of Comparative Example B3 is not describedbecause a half width in the XRD profile of Ketjen black was too large tobe calculated, and an electrical conducting material true specificgravity of Comparative Example B2 and Comparative Example B3 is notdescribed because the sample was too bulky to give reliable data.Furthermore, FIGS. 12 to 15 show the dispersing states of the finecarbon fiber of Example B1-6 with a pulverization time of 20 hours andelectrical conductivity assisting materials of Reference Example B1,Comparative Example B2 and Comparative Example B3, in the carbon-coatedlow-conductivity solid LiFePO₄, respectively. As seen in FIG. 12, thefine short carbon fiber of the present invention is evenly dispersed andadhere to the carbon-coated low-conductivity solid LiFePO₄ particles. InFIG. 13, there were a small number of fine carbon fibers which can beobserved in the surface of the carbon-coated low-conductivity solidLiFePO₄ particles. The fact indicates that short fine carbon fibersbefore fiber shortening are not dispersed but localized. In FIG. 14,acetylene black is substantially evenly dispersed, but a rate ofacetylene black adhering to the surface of the carbon-coatedlow-conductivity solid LiFePO₄ particles is lower than that in the fineshort fiber in FIG. 12 and dispersibility is also inferior to that. InFIG. 15, Ketjen black is also substantially evenly dispersed, but mostof Ketjen black particles are present in spaces between thecarbon-coated low-conductivity solid LiFePO₄ particles, and in terms ofeven dispersibility, it is inferior to the fine short carbon fiber inFIG. 12.

TABLE 2 Thickness of graphite stacked True specific plane gap of carbonlayers gravity of Surface area Ball milling conductive of conductiveconductive of conductive Experiment time material material materialmaterial name (hr) (nm) (nm) (g/cm³) (m²/g) Example B1-1 1 0.3425 2.72.06 255 Example B1-2 4 0.3425 2.8 2.06 257 Example B1-3 8 0.3426 2.92.06 258 Example B1-4 12 0.3427 3.0 2.07 260 Example B1-5 16 0.3430 3.22.07 261 Example B1-6 20 0.3432 3.2 2.07 261 Reference None 0.3424 2.72.05 250 Example B1 (fine carbon fiber) Comparative None 0.3370 2.8 — 69Example B2 (acetylene black) Comparative None 0.3468 — — 1365 Example B3(Ketjen black EC-600JD)

As seen in Table 2, a carbon layer gap of the fine carbon fiber was0.3424 nm, but by fiber shortening, a carbon layer gap of the fine shortcarbon fiber was slightly increased to 0.3432 nm (Examples B1-1 toB1-6). A thickness of the stacked carbon layers of the fine carbon fiberwas 2.7 nm, but by fiber shortening, a thickness of the stacked carbonlayers of the fine short carbon fiber was slightly increased to 3.2 nm(Examples B1-1 to B1-6). Furthermore, a true specific gravity of thefine carbon fiber was 2.05 g/cm³, while a true specific gravity of thefine short carbon fiber after fiber shortening was slightly increased to2.07 g/cm³ (Examples B1-1 to B1-6). Generally, it may be expected thatmechanical damage to a fiber is associated with fiber shortening,leading to considerable deterioration in carbon crystallinity in thefiber, but comprehensively judging from the above results, it can besaid that fiber shortening gives damage to carbon crystallinity verylittle or in a negligible level. Furthermore, increase in a surface areaassociated with fiber shortening is within a level supposed as atheoretical value. That is, it can be concluded that fiber shorteningproceeded to give a fine short carbon fiber, maintaining carboncrystallinity of the temple-bell-shaped structural unit aggregate in thefine carbon fiber before fiber shortening.

TABLE 3 Film Ball milling Film structure (parts by Film surfaceExperiment time weight) thickness resistance name (hr) S B F MF AB KJ(μm) (×10³Ω/□) Blank None 100 5 — — — — 58 13.5 Comparative ExampleExample B1-1 1 100 5 — 5 — — 75 6.2 Example B1-2 4 73 3.1 Example B1-3 870 1.4 Example B1-4 12 68 1.3 Example B1-5 16 65 1.0 Example B1-6 20 640.9 Reference None 100 5 5 — — — 127 8.4 Example B1 Comparative None 1005 — — 5 — 67 1.6 Example B2 Comparative None 100 5 — — — 5 71 2.2Example B3 Film structure S: Carbon coated LiFePO₄ B: Binder PVDF F:Fine carbon fiber MF: Fine short carbon fiber AB: Acetylene black KJ:Ketjen black EC-600JD

As seen in Table 3, in comparison with a film surface resistance of1.6×10³Ω/□ of acetylene black and a film surface resistance of2.2×10³Ω/□ of Ketjen black which have been conventionally used as anexcellent electrical conductivity assisting material, the fine shortcarbon fibers of Examples B1-5 and B1-6 have a surface resistance of1.0×10³Ω/□ and 0.9×10³Ω/□, respectively, indicating that a fine shortcarbon fiber produced by fiber shortening acts as a more effectiveelectrical conductivity assisting material. In Examples B1-1 to B1-6, asa pulverization time (a processing time by a ball mill) increased, afilm thickness was reduced. It is because as fiber shortening proceeds,entangled fine carbon fibers are shred and disentangled and becomes lessbulky and thus, a fine short carbon fiber after being shortened entersspaces between carbon-coated LiFePO₄ particles. In contrast, the finecarbon fiber used for Reference Example B1 exhibited inadequatedispersibility to be effective for promoting conductivity. It is becausethese carbon fibers form a secondary structure in which they areentangled each other, and in the dispersing method of this experiment,this secondary structure cannot be disentangled for dispersion.

Determination of a Particle Size Distribution

Variation in a fiber length depending on a ball milling time wasobserved using a particle size distribution measuring apparatus.

Preparation of a Sample for Determining a Particle Size Distribution

To an aqueous 0.015 wt % solution of Triton X-100 (dispersing agent,from Fisher Scientific Inc.) was added a fine carbon fiber or fine shortcarbon fiber to prepare a dilute slurry having a fiber concentration of0.001 wt %, which was then subjected to a ultrasonic treatment by anultrasonic dispersion apparatus for 4 min to prepare a dispersionliquid.

Apparatus: SALD-7000 particle size distribution measuring apparatus fromShimadzu Corporation, which uses a batch cell.

The measurement results are expressed as a relative particle amountdistribution as sphere, and a diameter of the sphere is taken as fiberlength.

FIG. 16( a) to (d) show ball milling times and variations in a particlesize distribution. The distribution varied as follows depending on aball milling time. In FIG. 16, (a) to (d) correspond to samples beforeball milling, after 6-hour ball milling, after 12-hour ball milling andafter 24-hour ball milling, respectively.

TABLE 4 Ball milling time 25% D 50% D 75% D (a) Before ball milling0.276 μm 0.358 μm 0.547 μm (b) 6 hrs 0.180 μm (c) 12 hrs 0.182 μm (d) 24hrs 0.132 μm 0.162 μm 0.197 μm

INDUSTRIAL USABILITY

There are provided a fine carbon fiber and/or a fine short carbon fiberthat have/has improved dispersibility and kneading properties in forminga composite with a polymer and/or a powder, which in turn attains acomposite having excellent processability, and allows a compositeexhibiting excellent functions such as conductivity, thermalconductivity, slide properties and reinforcement, as well as a processfor efficiently manufacturing them.

The invention claimed is:
 1. A process for manufacturing a fine carbonfiber comprising: feeding a mixed gas containing CO and H₂ onto acatalyst to initiate a reaction for growing the fine carbon fiber, thecatalyst containing an oxide of cobalt having a spinel crystal structurein which magnesium is contained by substitution forming solid solution,wherein the fine carbon fiber comprises a graphite plane consistingsolely of carbon atoms that form a temple-bell-shaped structural unithaving a body with two ends, one open and one closed, wherein an angleformed by a generatrix of the body and a fiber axis is less than 15°,wherein 2 to 30 of the temple-bell-shaped structural units are stackedsharing a common central axis to form multiple aggregates each having ahead and a tail, and wherein the head of at least one aggregate isconnected to the tail of at least one other aggregate to form the fiber.2. The process for manufacturing a fine carbon fiber according to claim1, wherein the oxide of cobalt having a spinel crystal structure isrepresented by Mg_(x)Co_(3-x)O_(y), in which “x” represents a solidsolution range of magnesium of 0.5 to 1.5.
 3. The process formanufacturing a fine carbon fiber according to claim 1, wherein in theoxide of cobalt having a spinel crystal structure, a crystal latticeconstant “a” (cubic system) is 0.811 to 0.818 nm as determined by X-raydiffraction measurement.
 4. The process for manufacturing a fine carbonfiber according to claim 1, wherein a volume ratio of CO/H₂ in the mixedgas is within the range of 70/30 to 99.9/0.1 and a reaction temperatureis within the range of 400 to 650° C.
 5. The process for manufacturing afine carbon fiber according to claim 1, wherein an end of the body of atleast one aggregate has an outer diameter D of 5 to 40 nm, an innerdiameter d of 3 to 30 nm, and an aspect ratio (L/D) of 2 to
 30. 6. Theprocess for manufacturing a fine carbon fiber according to claim 1,wherein an ash content of the fine carbon fiber is no more than 4% byweight.
 7. The process for manufacturing a fine carbon fiber accordingto claim 1, wherein a peak half width W (unit: degree) of 002 plane inthe fine carbon fiber is 2 to 4 as determined by X-ray diffractometry.8. The process for manufacturing a fine carbon fiber according to claim1, wherein a graphite plane gap (d002) of the fine carbon fiber is 0.341to 0.345 nm as determined by X-ray diffractometry.
 9. A process formanufacturing a fine short carbon fiber, comprising preparing a finecarbon fiber by the manufacturing process according to claim 1 and thenapplying shear stress to the fiber for shortening.